General Characteristics of Viruses

Viruses are infectious agents with both living and nonliving characteristics. They can infect animals, plants, and even other microorganisms. Viruses that infect only bacteria are called bacteriophage and those that infect only fungi are termed mycophage.

1. Living characteristics of viruses

a. They reproduce at a fantastic rate, but only in living host cells.

b. They can mutate.

2. Nonliving characteristics of viruses

a. They are acellular, that is, they contain no cytoplasm or cellular organelles.

b. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.

c. They possess DNA or RNA but never both.

3. Criteria used to define a virus

a. They contain only one type of nucleic acid: DNA or RNA, but not both.

b. They are totally dependent on a host cell for replication. (They are strict intracellular parasites.)

c. Viral components must assemble into complete viruses (virions) to go from one host cell to another.

4. Laboratory cultivation of viruses

Since viruses lack metabolic machinery of their own and are totally dependent on their host cell for replication, they cannot be grown in synthetic culture media. Animal viruses are normally grown in animals, embryonated eggs, or in cell cultures where in animal host cells are grown in a synthetic medium and the viruses are then grown in these cells.

Size and Shapes of Viruses

1. Size

Viruses are usually much smaller than bacteria and are submicroscopic. Most range in size from 5 to 300 nanometers (nm), although some Paramyxoviruses can be up to 14,000nm long.

2. Shapes

a. Helical viruses consist of nucleic acid surrounded by a hollow protein cylinder or capsid and possessing a helical structure

b. Polyhedral viruses consist of nucleic acid surrounded by a polyhedral (many-sided) shell or capsid, usually in the form of an icosahedron

c. Enveloped viruses consist of nucleic acid surrounded by either a helical or polyhedral core and covered by an envelope
d. Binal (complex) viruses have neither helical nor polyhedral forms, are pleomorphic (irregular shaped), or have complex structures

Viral Structure

Since viruses are not cells, they are structurally much more simple than bacteria. An intact infectious viral particle is called a virion and consists of:

1. A genome: the viral genome is a single or segmented, circular or linear molecule of nucleic acid functioning as the genetic material of the virus. It can be single-stranded or double-stranded DNA or RNA (but never both), and codes for the synthesis of viral components and viral enzymes for replication.

2. A capsid: the capsid, or core, is a protein shell surrounding the genome and is usually composed of protein subunits called capsomeres. The capsid serves to protect and introduce the genome into host cells. Some viruses consist of no more than a genome surrounded by a capsid and are called nucleocapsid or naked viruses.

3. An envelope: most animal viruses also have an envelope surrounding a polyhedral or helical nucleocapsid, in which case they are called enveloped viruses. The envelope is composed of phospholipids and glycoprotein and for most viruses, is derived from host cell membranes by a process called budding. The envelope may come from the host cell's nuclear membrane, vacuolar membranes (packaged by the Golgi apparatus), or outer cytoplasmic membrane.

Although the envelope is usually of host cell origin, the virus does incorporate proteins of its own, often appearing as glycoprotein spikes, into the envelope. These glycoprotein spikes function in attaching the virus to receptors on susceptible host cells. In order to protect against infection, one of the things the body must initially do is detect the presence of microorganisms. The body does this by recognizing molecules unique to microorganisms that are not associated with human cells.

Bacteriophages are viruses that only infect bacteria. Some bacteriophages are structurally much more complex than typical nucleocapsid or enveloped viruses and may possess a unique tail structure composed of a base plate, tail fibers, and a contractile sheath. Other bacteriophages, however, are simple icosahedrals or helical

Classification of Viruses

Viruses are often classified by the type of nucleic acid they have for their genome, the shape of their capsid (helical or polyhedral), and whether they are enveloped or naked (lack an envelope). Viruses can store their genetic information in six different types of nucleic acid which are named based on how that nucleic acid eventually forms the viral mRNA able to bind to host cell ribosomes and be translated into viral proteins. These types of nucleic acid are:

a. (+/-) double-stranded DNA. The (-) DNA strand is directly transcribed into viral mRNA. Most bacteriophages, Papovaviruses, Adenoviruses, Herpesviruses.

b. (+) DNA or (-) DNA. Once inside the host cell, its converted into dsDNA and the (-) DNA strand is transcribed into viral mRNA. Phage M13, Parvoviruses.

c. (+) RNA. A (+) RNA is copied into (-) RNA that is transcribed into viral mRNA. Picornaviruses, Togaviruses, Coronaviruses.

d. (-) RNA. The (-) RNA is copied into a (+) RNA which functions as viral mRNA. Orthomyxoviruses, Paramyxoviruses, Rhabdoviruses.

e. (+/-) double-stranded RNA. The (+) of the (+/-) RNA functions as viral mRNA. Reoviruses.

f. (+) RNA. The (+) RNA is reverse transcribed into (-) DNA that makes a complementary copy to become (+/-) DNA. The (-) DNA is transcribed into viral mRNA. Retroviruses.

Viroids and Prions

Viroids are even simpler than viruses. They are small, circular, single-stranded molecules of infectious RNA lacking even a protein coat. They are the cause of a few plant diseases such as potato spindle-tuber disease,cucumber pale fruit, citrus exocortis disease, and cadang-cadang (coconuts).

Prions are infectious protein particles thought to be responsible for a group of transmissible and/or inherited neurodegenerative diseases including Creutzfeldt-Jakob disease, kuru, and Gerstmann-Straussler- syndrome in humans as well as scrapie in sheep and goats. Most evidence indicates that the infectious prion proteins are modified forms of normal proteins coded for by a host gene in the brain.

Life Cycle of Bacteriophages

There are two primary types of bacteriophages: lytic bacteriophages and lysogenic (temperate) bacteriophages.

1. Bacteriophages that replicate through the lytic life cycle are called lytic bacteriophages, and are so named because they lyse the host bacterium as a normal part of their life cycle.

2. Bacteriophages capable of a lysogenic life cycle are termed temperate phages. When a temperate phage infects a bacterium, it can either replicate by means of the lytic life cycle and cause lysis of the host bacterium, or, it can incorporate its DNA into the bacterium's DNA and become a noninfectious prophage.


Viral Infections of Humans

Most viruses that infect humans, such as those that cause routine respiratory infections (e.g., cold viruses, influenza viruses) and gastrointestinal infections (e.g., Rotaviruses, Norwalk virus), cause acute infections. Acute infections are of relatively short duration with rapid recovery.

In persistent infections, the viruses are continually present in the body. Other persistent infections are known as latent viral infection. In a latent viral infection the virus remains in equilibrium with the host for long periods of time before symptoms again appear, but the actual viruses cannot be detected until reactivation of the disease occurs. Examples include infections caused by HSV-1 (fever blisters), HSV-2 (genital herpes), and VZV (chickenpox-shingles).

In the case of chronic virus infections, the virus can be demonstrated in the body at all times and the disease may be present or absent for an extended period of time. Examples include hepatitis B (caused by HBV) and hepatitis C (caused by HCV).

Slow infections are ones in which the infectious agents gradually increase in number over a very long period of time during which no significant symptoms are seen. Examples include AIDS (caused by HIV-1 and HIV-2) and certain lentiviruses that cause tumors in animals. Although not viruses, prions also cause slow infections.